Historically, commercial quantities of potassium chlorate have been produced by the double decomposition of sodium chlorate and potassium chloride; EQU NaClO.sub.3 +KCl.fwdarw.NaCl+KClO.sub.3
The sodium chlorate used in this process has ordinarily been produced directly by the electrolysis of an aqueous sodium chloride solution in an electrolytic cell. To each batch of sodium chlorate produced potassium chloride is added stoichiometrically; the resulting KClO.sub.3 /NaCl solution is cooled; and the KClO.sub.3 crystals that form are separated from the solution. The industry practice has been to boil down the remaining solution, or mother liquor, to adjust the water concentration to the level employed in the electrolytic cell and to return the concentrated liquor to the cell for further electrolysis with the NaCl added by the above reaction to produce more sodium chlorate according to the reaction EQU NaCl+3H.sub.2 O.fwdarw.NaClO.sub.3 +3H.sub.2 .uparw..
Since the separation of KClO.sub.3 is not 100% efficient, potassium ions will inevitably be present in the concentrated liquor returned to the cell, necessitating the operation of the cell at high temperatures to prevent the crystallization of the potassium. These high temperatures and the potassium ions present cause very rapid wear results in high equipment costs, while labor costs are elevated by the fact this process is carried out in a batch, rather than on a continuous, basis.
U.S. Pat. No. 3,883,406, the disclosure of which is fully incorporated herein by reference, is directed to a process for recovering electrolytically produced alkali metal chlorates obtained by the direct electrolysis of sodium chloride to sodium chlorate in diaphragmless cells equipped with dimensionally stable anodes of a valve metal, such as titanium, coated with a noble metal and/or oxide thereof. The discussion of the prior art in this patent explains that NaCl is less soluble than NaClO.sub.3 at the temperatures conventionally used, so that during the concentration and evaporative cooling steps of the prior art, NaCl crystals separate from the cell liquor first and are removed by filtration or centrifugation. This NaCl may then be redissolved and returned to the cell. U.S. Pat. No. 3,883,406 itself discloses processes wherein solutions are achieved having chlorate concentrations in excess of 700 grams NaClO.sub.3 per liter and chloride concentrations as low as 40 grams NaCl per liter. At the high chlorate/chloride concentrations obtained, evaporative cooling causes the chlorate to crystallize first if sufficient vacuum is applied. The particular advantages of the process disclosed in U.S. Pat. No. 3,883,406 are achieved by electrolyzing the NaCl solution to produce a ratio of NaClO.sub.3 :NaCl of at least 5:1 and preferably at least 7:1.
When the direct electrolysis of alkali metal chlorides to alkali metal chlorates in aqueous solution is carried out, chlorine is produced at the anode while alkali metal hydroxide forms at the cathode. The chlorine and hydroxyl ions are thus free to react chemically to form alkali metal hypochlorite, as is shown by the following equation illustrating the process with potassium: ##STR1## The hypochlorite rapidly converts to form chlorate; EQU 3 KClO.fwdarw.2 KCl+KClO.sub.3
The reversible nature of the formation of alkali metal hypochlorite accounts for significant process inefficiencies where oxygen is liberated into the cell liquor when the hypochlorite decomposes instead of disproportionating into the chloride and the chlorate. Prior to the advent of metal anodes, the direct production of potassium chlorate was uneconomical because the low solubility of KClO.sub.3 in water at the temperature previously employed (e.g. 4-5% in H.sub.2 O at 30.degree. C.) limited the recovery of KClO.sub.3 when compared with the yields available in the conventional double decomposition process.
U.S. Pat. No. 4,046,653 discloses a process for producing sodium or potassium chlorate by the direct electrolysis of the corresponding chloride at temperatures of 90.degree.-110.degree. C. The working example that discloses the electrolysis of potassium chloride starts with a solution containing 300 g per liter of solution as a starting electrolyte, achieving concentrations of 90 g/l potassium chloride and 210 g/l potassium chlorate at steady state operating conditions. While this patent discloses the discharge of an equal volume of electrolyte from the cell as the KCl brine is fed in, we have determined that it is not possible to operate a closed loop process in accordance with this patent using only a saturated brine without adding additional solid KCl directly to the cell electrolyte and that the results stated are not significantly different from those expected from the electrolysis of sodium chloride. In contrast, our process produces surprising results in efficiency increases not accounted for by the heavier weight of potassium chlorate.